Witnesses may not testify to opinions concerning intent, guilt, or innocence in a criminal case; the truth or falsity of allegations; whether a witness has testified truthfully; or legal conclusions.

So, if such testimony is offered and opposing counsel fails to object, would the admission of such testimony constitute plain error necessitating a new trial? According to the recent opinion of the Court of Appeals of Indiana in Gutierrez v. State, 2012 WL 560048 (Ind.App. 2012), the answer is "yes."

In Gutierrez, Ernesto Gutierrez was charged with two counts of Child Molesting. After he was convicted, Gutierrez appealed, claiming, inter alia, that the trial court erred by allowing for the admission of testimony by sexual assault nurse Michelle Ditton and Penny Hasselman, a case manager with the Department of Child Services, both of whom interviewed M.L., the alleged victim. At trial,

when Ditton was asked about whether she believed M.L., counsel objected on the grounds that such testimony would damage "the province of the jury as to whether she's telling the truth or not."...The trial court overruled the objection, and responded, "I agree, but I think it's up to the jury to give that opinion whatever weight it deems appropriate, so I'll overrule that objection."....Ditton then responded as follows:

I believe based on the time frame since the last occurrence, based on the fact that she told me she never had any bleeding, based on the fact that it felt good that it probably was—when the penis was on the clitoris, based on the fact of how easily, even if there was minor injury to that tissue, based on the fact, again, probably the most important that that tissue is very estrogenized thick, could easily accommodate a speculum, a penis, a baby's head, I didn't expect to find any injury before I even looked at her.

Later,

the deputy prosecutor asked Hasselman: "With your time being spent with [M.L.], and hearing what happened in the deposition, did you believe what she was saying?"...Hasselman responded, "absolutely."...The deputy prosecutor then asked if Hasselman could explain why she believed M.L., and Gutierez's counsel objected on the grounds of relevance....The deputy prosecutor then remarked that "I think [the jury] should hear why she believes from her past experience."... The trial court then sustained defense counsel's objection to this comment on the grounds of relevance.

The Court of Appeals found error with the admission of Ditton's testimony, but it is unclear whether that error alone was enough to reverse. The court also found error with the admission of Hasselman's testimony and found that Gutierrez had failed to preserve the issue by failing to object under Rule 704(b). That said, the court then found that

When examining the exchange between the deputy prosecutor and Hasselman, it is readily apparent that the provisions of Indiana Rule of Evidence 704(b) were violated, which resulted in an invasion of the province of the jury to judge the credibility of the witnesses. As noted above, Hasselman testified that she "absolutely" believed M.L.'s testimony....And the deputy prosecutor contemporaneously inserted his own opinion that he believed M.L. Therefore, the admission of Hasselman's testimony amounted to fundamental error.